The present invention relates to a self-priming pump in which a pump casing is formed with an intake-side priming fluid chamber communicating with an inlet port, a discharge-side priming fluid chamber communicating with an outlet port, and a pump chamber containing an impeller therein.
In self-priming pumps of this type, when the pump drive is stopped, a pump fluid ceases to flow in a feeding direction and is sucked to the intake side by a difference in pressure between the intake and discharge sides, thus causing a backflow. In this case, a priming fluid to be secured in the priming fluid chambers may possibly flow out of the pump casing, urged by the backflow caused by siphonage on the intake side.
In order to avoid this, a communication hole or orifice is bored through a partition member which divides the intake-side priming fluid chamber from the discharge-side priming fluid chamber, the hole corresponding in position to the level of the priming fluid to be maintained. Thus, when the surface level of the fluid lowers to reach the level of the communication hole when the fluid flows backward, the two priming fluid chambers communicate with each other by means of the hole. As a result, the effect of siphonage is canceled to maintain the predetermined priming fluid level. This arrangement is already disclosed in, for example, Japanese Patent Publication No. 59-8675.
Although the communication hole is a small orifice, however, a circulating flow passes through it to cause a leakage of pressure between the intake and discharge sides during pumping operation, since the hole is left open at all times. Consequently, the discharge pressure and discharge rate are reduced to lower the pump efficiency. Also during self-priming operation for the start of the pumping operation, the intake-side negative pressure cannot sufficiently be increased due to the pressure leakage through the communication hole. Thus, the efficiency of the self-priming operation would be lowered.
In an alternative prior art arrangement of the self-priming pump, a non-return valve is provided near an inlet port. During the pumping operation, in this case, the valve opens to allow the fluid to flow in the feeding direction. When the pumping operation is stopped, the valve is closed to prevent the fluid from flowing out through the inlet port if a backflow is caused by siphonage on the intake side. In this manner, the priming fluid is secured in the pump. With this arrangement, however, the position of the inlet port must be aligned with the surface level of the priming fluid to be maintained, lowering the degree of freedom of the layout of the inlet port.